PPS has excellent characteristics such as a high heat resistance, high dimension stability, and low melt viscosity. However, ordinary PPS contains sodium chloride and other alkali metal compounds resulting from the preparation process as impurities, and therefore, the product does not have a reliable moisture resistance and the use thereof is considerably restricted even though the use of PPS as an encapsulant for IC or LSI is desired.
Various methods for removing impurities such as alkali metal compounds from PPS have been proposed. For example, Japanese Unexamined Patent Publication No. 57-108,135 discloses a method in which PPS is treated with a mixed solvent comprising ethylene glycol and N-methyl-2-pyrrolidone; Japanese Unexamined Patent Publication No. 57-108,136 discloses a method in which a chemical such as sodium carbonate is added to an aqueous slurry of PPS and the mixture is treated at a high temperature under a high pressure; and Japanese Unexamined Patent Publication No. 59-219,331 discloses a method in which PPS is heat-treated in an aromatic solvent such as diphenyl ether, diphenyl, 1-chloronaphthalene, p-chlorophenol or benzonitrile.
In the method where PPS is treated with a mixed solvent of ethylene glycol and N-methyl-2-pyrrolidone, a large quantity of the solvent must be used and the same operation must be repeated at least two times, preferably 3 to 5 times, for removing impurities to a required level. The effect of removing impurities is not sufficient in the method where sodium carbonate is added to an aqueous slurry of PPS and the mixture is treated. Moreover, the method in which PPS is heat-treated in an aromatic solvent such as diphenyl ether or diphenyl has a problem in that, since the melting point of the solvent is relatively high, the treatment must be carried out at a high temperature and separation of the solvent is relatively difficult.
It is an object of the present invention to provide a method for removing impurities such as alkali metal compounds from PPS with a high efficiency.